1. Field of the Invention
This invention relates to magnetic bubble memories.
2. Description of the Prior Art
Magnetic bubble memories are well known in the art. The most familiar type of bubble memory is one in which a pattern of magnetic elements is formed adjacent the surface of a film in which the bubbles are moved. The elements are designed such that a repetitive pattern of magnetic poles move along channels defined by the elements in response to a magnetic drive field reorienting in the plane of bubble movement. The elements, generally, are composed of magnetically soft permalloy and are in T and bar shapes in the most familiar circuits. Most recently, asymmetric half-disc shaped elements are being used.
Magnetic bubbles are of circular or disc-shape when observed under a microscope through an analyzer in the presence of polarized light as is well known. The diameter of the disc is determined by a bias field antiparallel to the magnetization of the bubble. Typically, an epitaxially grown garnet film defines the plane of bubble movement and the magnetization of the film is normal to the plane. Thus, the bias field is oriented normal to the plane of bubble movement in a direction antiparallel to the magnetization of the bubble. The reorienting (typically rotating) magnetic in-plane field can be understood to generate pole patterns in the permalloy elements. These pole patterns, in turn, modify the bias field causing localized magnetic field gradients in the film resulting in bubble movement. Due to the fact that bubble movement is caused by a magnetic field rather than by an arrangement of electrical conductors and due to the fact that the permalloy elements are operative to move those bubbles to a detector for accessing information represented by the bubble pattern, this form of bubble memory is commonly called a "field access" bubble memory.
A field access bubble memory is commonly organized in a "major-minor" configuration. The term "major-minor" describes an organization in which the pattern of permalloy elements defines a plurality of closed loop paths, termed "minor" loops, about which bubble patterns move and an accessing path called the "major" path. The plurality of minor loops is operative as a permanent store. Information in the form of a bubble pattern is moved in parallel from the permanent store to the accessing path at reference positions defined typically by a "turn" element in each minor loop. To this end, an interchannel, bubble movement-controlling, permalloy pattern is formed between the turn element of each minor loop and an associated stage of the accessing channel. An electrical conductor couples the "interchannel" permalloy pattern and/or turn elements causing the information, represented by the bubble pattern, to be moved between channels when the conductor is pulsed.
The information can be moved between channels in a variety of modes. In one of these modes, bubble transfer occurs. In this mode, the interchannel permalloy patterns and turn elements define bubble "transfers" operative to move information from the minor loops to the major channel leaving vacancies at the turn elements. In this form of bubble memory, the major channel is in the form of a recirculating loop also and the number of stages in the major loop is related to the number in each minor loop such that as the in-plane field continues to rotate, the transferred information and the vacancies from which the information originated move about the various loops and arrive at the interchannel pattern simultaneously. A second pulse later applied to the transfer conductor is operative to restore the transferred information to the originating positions.
An alternative mode of operation for bubble memories results when a plurality of bubble replicators are employed. The replicators are operative to produce an image of the bubble pattern, for example, at the turn elements in the minor loops for movement to the major channel. Again, an electrical conductor couples the interchannel patterns and/or turn elements electrically in series for controlling the replication operation. This form of bubble memory has the virtue that image information need not be returned to the originating vacancies, thus leading to faster accessing times.
A bubble replicator operates by stretching a bubble into an elongated form and by generating a cutting field in the middle of the so-stretched bubble causing separation into two. The constraints to this operation arise primarily from the bias field and the in-plane field. The constraint due to the bias field arises from the fact that every bubble memory exhibits stable operation over a range of bias field values from a low value at which bubble strip-out occurs to a high value at which bubble collapse occurs. Although operation occurs over a very generous range, bubble elongation is reduced at the high bias field end of the range. The constraint due to the in-plane field arises from the fact that bubble elongation and cutting occurs over only a portion of a cycle of the in-plane field. The higher the frequency of the field, the greater the constraint on the replicator. Naturally, if the replicator were able to operate over an increased portion of an in-plane field cycle, those constraints due to the in-plane field would be reduced.
An increase in the bulk of the permalloy of the turn element also would be a benefit. A properly designed pattern would produce increased pole strength which would operate to elongate the bubble in spite of high bias field values. But, increased pole strength is achieved at the expense of elongated features in the turn element geometry and these features always seem to produce unwanted poles in unwanted positions and to occupy increasing amounts of precious film surface area.